Digital magnetic recording stores digital data by modulating a magnetic flux pattern on a magnetic medium. During the storing process, an electric current in a write head is modulated based on the digital data to be written. The head is positioned over magnetic material in the shape of a circular disk, which rotates rapidly. The electric current in the write head, in turn, modulates the magnetic flux pattern on the medium. The medium used is such that the flux pattern is retained in the medium after the electric current is turned off in the write head, thus providing data storage.
Data is usually written on the medium in concentric circles called tracks, which are further divided into user or read data sectors and servo sectors embedded between the read data sectors. The servo sectors contain data and supporting bit patterns required for control and synchronization. The control and synchronization information is used to position the magnetic recording head on track, so that the information stored in the read data sectors can be retrieved properly.
During a read process, a read head is positioned over the medium following the tracks, but now the magnetic flux pattern on the medium induces a current in the read head. This current is then processed to recover the written data. More specifically, an analog signal path (including filtering, amplification, and timing stages) processes the signals from the read head. The read process begins with reading synchronization bit patterns containing, for example, preamble symbols, a servo address mark, and a number of other fields that are used for head positioning and other purposes, such as providing track number and sector information.
Read heads, such as, magnetoresistive read heads, may produce signals having asymmetric properties when reading the data, including servo data. Since the servo sector data is a relatively short data field, traditional techniques to compensate for signal asymmetry using least mean squared error adaptive techniques, for example, would typically fail to adequately compensate for asymmetry in the servo data. The least mean squared error adaptive techniques are more suited for long non-periodic data patterns for proper operation.